[asterisk-users] Got event 17 (Polarity Reversal)...

[Wilton Helm]

>You should not get that message on analog lines in the USA or Canada.  I 
>suspect your line has a provisioning issue or is using different 
>signaling than you think it is using.


Not necessarily true.  Most recent solid state switches have abandoned this as a cost saving measure.

Polarity reversal was originally done on all loop start and ground start circuits on the calling end when the called party answered.  It was used to identify start of call.  On a relay based step switch it was fairly easy to do, because the path went through a relay contact that changed at that point anyway, so it was just deliberately wired backwards.

It created havoc with early DTMF pads, because they used transistors and needed a known polarity.  That was solved by adding a bridge rectifier in the dialpad.

Loop and Ground Start trunks can still be ordered with this option because it helps accurately determine start of call for CDR.  However, it requires a more expensive line card in the CO, so they don't like to do it.  A similar switch when a call is first terminated (sometimes before it rings) can be used to insure that a line is not accidentally picked for an outgoing call by a PBX when a call is coming in on it.

Wilton
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